As classes make their first visits of the school year to my lab I try to give the kids something really exciting to work on to get them interested in Science. Last year I used Oobleck, which I couldn't use again due to the fact my 3rd graders had already seen it, so this year I went with something similar called glurch.
I had originally used this recipe, which called for either a mix of 50% water and 50% white glue OR water and boric acid, but I have found a combination of both works really well.
I let the kids mix the water and glue mix together, give them a discussion about the properties of matter and then have them mix the boric acid, which can be found in the laundry detergent aisle under the name "20 Mule Team Borax". You should have kids wear safety gear while creating the mix, as borax is an eye irritant. The mixture, when just right, will bounce like a superball and take permanent marker off of objects.